Raman dispersion spectroscopy probes heme distortions in deoxyHb-trout IV involved in its T-state Bohr effect

Abstract

The depolarization ratios of heme protein Raman lines arising from vibrations of the heme group exhibit significant dependence on the excitation wavelength. From the analysis of this depolarization ratio dispersion, one obtains information about symmetry-lowering distortions deltaQ(Gamma) of the heme group that can be classified in terms of the symmetry races Gamma = A(1g), B(1g), B(2g), and A(2g) in D(4h) symmetry. The heme-protein interaction can be changed by the protonation of distinct amino acid side chains (i.e., for instance the Bohr groups in hemoglobin derivates), which gives rise to specific static heme distortions for each protonation state. From the Raman dispersion data, it is possible to obtain parameters by fitting to a theoretical expression of the Raman tensor, which provide information on these static distortions and also about the pK values of the involved titrable side chains. We have applied this method to the nu(4) (1,355 cm(-1)) and nu(10) (1,620 cm(-1)) lines of deoxygenated hemoglobin of the fourth component of trout and have measured their depolarization ratio dispersion as a function of pH between 6 and 9. From the pH dependence of the thus derived parameters, we obtain pK values identical to those of the Bohr groups, which were earlier derived from the corresponding O(2)-binding isotherms. These are pK(alpha1) = pK(alpha2) = 8.5 for the alpha and pK(beta1) = 7.5, pK(beta2) = 7.4 for the beta chains. We also obtain the specific distortion parameters for each protonation state. As shown in earlier studies, the nu(4) mode mainly probes distortions from interactions between the proximal histidine and atoms of the heme core (i.e., the nitrogens and the C(alpha) atoms of the pyrroles). Group theoretical argumentation allows us to relate specific changes of the imidazole geometry as determined by its tilt and azimuthal angle and the iron-out-of-plane displacement to distinct variations of the normal distortions deltaQ(Gamma) derived from the Raman dispersion data. Thus, we found that the pH dependence of the heme distortions deltaQ(A1g) (totally symmetric) and deltaQ(B1g) (asymmetric) is caused by variations of the azimuthal rather than the tilt angle of the Fe-His (F8) bond. In contrast to this, the nu(10) line mainly monitors changes resulting from the interaction between peripheral substituents of the porphyrin macrocycle (vinyl). From the pH dependence of the parameters, it is possible to separately identify distortions deltaQ(Gamma) affecting the hemes in the alpha and beta chains, respectively. From this, we find that in the alpha subunit structural changes induced on protonation of the corresponding Bohr groups are mainly transferred via the Fe-N(epsilon) bond and give rise to changes in the azimuthal angle. In the beta subunit, however, in addition, structural changes of the heme pocket arise, which most probably result from protonation of the imidazole of the COOH-terminal His (HC3 beta). This rearranges the net of H bonds between His HC3 beta, Ser (F9 beta), and Glu (F7 beta).